[Elecraft] 12 meter K3 RFI from nearby FM station

[Vic Rosenthal 4X6GP/K2VCO]

Jon,

I had a similar problem with a 50 kW AM BC station a few miles away. It 
turned out to be strong enough to override the bias in the K3's T/R 
switch. I built a high-pass filter which solved the problem.

You would need a low-pass filter in your case. Make sure it can handle 
100w, because you must connect it in the main antenna path -- the RX 
in/out path is before the T/R switch.

73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/

On 10 Feb 2016 16:58, Jon Zaimes wrote:
> I had been noticing some RFI on 12 meters for some time that I initially had
> thought was power line noise. My use of 12 meters is seldom so it wasn't
> very high up on the list of things to track.
>
> When I was reminded of it while listening for VP8STI recently, I did a
> little more snooping and realized it was actually splatter from a BC signal
> at the low edge or slightly below the 12 meter band, centered about 24.89
> mHz but audible from 24.87-24.91 mHz.
>
> The distorted splatter is audible as a BC station, all talk, no music, but
> not clear enough to decipher any ID.
>
> A few days ago I decided to try to pinpoint the source, so listened to the
> RFI on one ear while scanning the AM and SW bands with the sub RX on the
> other ear. But I could find no matches, though did find the splatter popped
> up centered on these additional frequencies:
>
> 813 kHz
> 996 kHz
> 2.820 mHz
> 3.626 mHz
> 19.4 mHz
>
> Figuring it must be an FM BC station, I started going through a list of
> Delaware FM stations and on the third one, 91.1 mHz, matched the audio peaks
> to my splatter. Further research determined this was a public radio station
> with a transmitter on a commercial tower just a half mile west of my QTH!
> While I of course was aware of the tower, which I knew to carry a couple of
> cell sites and trunking radio, I hadn't been aware of the FM station, which
> has apparently been there about 2 years. FCC data shows it has 2.1 KW ERP. I
> had not had any previous issues with the site over the past 17 years.
>
> Thinking that mixing or rectification might be produced either at the TX
> site or on my own QTH, I decided to track with my mobile rig, an IC-706
> MK2G. But there was NO evidence of the splatter on the 706.
>
> Further debugging found that I was hearing the same splatter on BOTH of my
> K3 radios (no. 3021 and 3057). Even when ALL cables are removed (only the
> power lead connected). Grounding or ungrounding made no change. I tried the
> radios on a battery but still had the RFI. Tried some ferrites on the power
> lead but no change.
>
>
> So I'm thinking now there's some internal mixing in the K3 going on, and
> wondering if any others have had similar issues and found any solution.
>
> 73/Jon AA1K
> Felton, Delaware
> www.aa1k.us
>
>
>
>
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